The present invention relates to a firing system for a thermal cracking furnace and, more particularly, a firing system for a thermal cracking furnace of the type having hearth burners for heating radiant walls of the furnace to thereby effect heating by radiation of radiant coils.
A thermal cracking furnace thermally cracks a hydrocarbon feedstock such as naphtha, ethane, and propane during circulation of the feedstock through radiant coils suspended within the furnace. Coke and tar products produced by pyrolysis of the hydrocarbon feedstock during the cracking process leads to eventual fouling of the radiant coils, necessitating cleaning (decoking) or replacement of the coils. The selectivity, yield, and run lengths between decoking cycles are typically closely related to the heat flux profile along the vertical extent of the radiant coils.
The thermal cracking process involves combustion of a fossil based fuel such as gas in a manner which unavoidably results in the creation of NO.sub.X. NO.sub.X emissions are recognized to be a significant source of air pollution. Thus, environmental emissions standards have been and continue to be imposed by various governmental authorities which limit the amount of NO.sub.X gases which can be emitted into the atmosphere. Several designs have been proposed to inhibit the production of NO.sub.X gases including designs which limit the production of NO.sub.X gases due to the mixing of fuel and furnace or flue gases for combustion in a combustion zone of a thermal cracking furnace. Although these designs may be advantageous, there still remains the need for burner designs for a thermal cracking furnace having improved NO.sub.X reduction characteristics.